1. Field of the Invention
The present invention relates to a photothermographic material, and in particular, it relates to a photothermographic material using a silver halide emulsion having a high silver iodide content and to a photothermographic material that is significantly improved in sensitivity, is low in fogging, and is excellent in storage stability of an image after development.
2. Description of the Related Art
In recent years, a dry development process has been sought in the medical field and the printing and prepress field from the standpoint of environment protection and space saving. In these fields, digitalizatio has progressed, and sysytems in which image information is imported to a computer, stored, and then processed have widely spread. Such systems are advantageous since image information can be communicated to a desired location and then output to a photosensitive material, which is developed to form an image. The photosensitive material is required to have such characteristics that recordation can be attained by exposure with laser light having a high illuminance to form a clear black image having high resolution and sharpness. For use as the digital imaging recording material, various kinds of hardcopy systems utilizing pigments and dyes, such as an ink-jet printer and an electrophotographic system, have been commercially available as a general image forming system. However, they are insufficient in image quality (such as sharpness, graininess, gradation and color tone) required for diagnostic medical images and in recording rate (sensitivity).
A thermal image forming system utilizing an organic silver salt has been described in the literature. In particular, a photothermographic material generally has an image forming layer containing a photosensitive silver halide, a reducing agent, a reducing silver salt (such as an organic silver salt), and depending on necessity, a color toning agent for controlling the color tone of silver, which are dispersed in the matrix of a binder.
After subjecting a photothermographic material to imagewise exposure, it is heated to a high temperature (for example, 80° C. or more) to form a black silver image through a redox reaction between the silver halide or the reducing silver salt (which functions as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by catalytic action of a latent image of a silver halide formed through exposure. As a result, a black silver image is formed on an exposed area. As a practical application for a medical image forming system, FUJI MEDICAL DRY IMAGER FM-DP L has been commercially available.
The image forming system utilizing an organic silver salt has a significant problem in storage stability of an image, particularly a tendancy to cause a printout upon irradiation with light, due to the absence of a fixing process. As a measure for improving a printout, U.S. Pat. No. 6,143,488 and EP-A 0,922,995 disclose methods of utilizing silver iodide formed by converting an organic silver salt. Thus, a printout has been improved by using silver iodide.
However, the methods thus disclosed, where an organic silver salt is converted with iodine, cannot provide sufficient sensitivity, and thus it is difficult to constitute a practical system. Photosensitive materials utilizing silver iodide are also described in International Publication Nos. 97/48014 and 97/48015, U.S. Pat. No. 6,165,705, JP-A No. 8-297345 and Japanese Patent No. 2,785,129, but they cannot attain sufficient sensitivity and fogging property and thus cannot be practically used as a photosensitive material for laser exposure.
The utilization of silver iodide brings about other problems in addition to that in sensitivity. For example, there are such problems specific to silver iodide in that the gradation becomes high contrast, and the in that there is a tendancy for sensitivity to be lowered prior to use due to deteriorated storage stability.